Move setting SELinux booleans to platform code
    
    Create a platform task for setting SELinux booleans.
    
    Use an exception for the case when the booleans could not be set
    (since this is an error if not handled).
    Since ipaplatform should not depend on ipalib, create a new
    errors module in ipapython for SetseboolError.
    
    Handle uninstallation with the same task, which means
    the booleans are now restored with a single call to
    setsebool.
    
    Preparation for: https://fedorahosted.org/freeipa/ticket/4157
    
    Fixes: https://fedorahosted.org/freeipa/ticket/2934
    Fixes: https://fedorahosted.org/freeipa/ticket/2519
    Reviewed-By: Thierry Bordaz <tbordaz@redhat.com>